1. Field of the Invention
The present invention relates to an internal combustion engine controller, and more particularly to an internal combustion engine controller provided with a unit for preventing diffusion of fuel vapor produced in a fuel supply system of automobiles.
2. Description of the Related Art
A self-diagnosis device for use in a unit for preventing diffusion of fuel vapor is disclosed in Japanese Patent Laid-Open No. 2-130255. The disclosed device has a pressure sensor disposed in a supply passage connecting a canister and an intake pipe. Based on the result detected by the pressure sensor, the device detects such an abnormality in fuel supply that no fuel vapor is supplied to the intake pipe.
As disclosed in Japanese Patent Laid-Open No. 2-136558, there is also known a device designed to detect the generation of fuel vapor by measuring the pressure in a fuel tank, open and close a purge control valve upon the generation of fuel vapor, and detect an abnormality based on a deviation of the air/fuel ratio at that time.
With the device of the above-cited Japanese Patent Laid-Open No. 2-130255, however, the pressure detection by the pressure sensor disposed in the supply passage enables an extreme abnormality such as disconnection or clogging of pipes to be detected, but has difficulties in detecting a reduction in the passage area due to dust deposits in pipes, a lowering of the flow rate in the purge control valve due to malfunction of a valve body of the purge control valve, suction of the open air due to cracks of pipes or other causes, etc. Such a change in flow rate characteristics (or such a lowering of the flow rate purged) would result in that a sufficient supply ability (purging ability) of the fuel vapor from the canister can no longer be ensured. This will bring activated charcoal in the canister into a broken state sooner or later (beyond the adsorption capacity), causing the fuel vapor to be discharged through a hole of the canister open to the atmosphere.
With the device of the above-cited Japanese Patent Laid-Open No. 2-136558, the deviation of the air/fuel ratio is largely fluctuated depending on the amount of air remaining in a fuel tank. For example, when the amount of fuel in the fuel tank is large, even with a small amount of the fuel vapor the pressure is so raised as to satisfy conditions for detecting an abnormality. In this case, since the fuel vapor is lean and the air/fuel ratio remains unchanged, the fuel vapor diffusion preventing unit, even though it is under normal condition, is judged to be abnormal and false detection results. Also, at the time fuel begins to vaporize, the air purged into an intake pipe becomes so lean that the air/fuel ratio is not changed even when the purge control valve is opened and closed. Therefore, in spite of being normal, the fuel vapor diffusion preventing unit is judged to be abnormal and false detection results. Further, when density of the fuel vapor is rich, the air/fuel ratio is changed even in the event there occur cracks or the like in part of pipes. Consequently, in spite of being abnormal, the fuel vapor diffusion preventing unit is judged to be normal and false detection results.
In addition, duty - flow rate characteristics of the purge control valve are varied to a large extent, particularly in the range of low flow rates, due to tolerance in manufacture, changes over time and other causes. This gives rise to the problem that the purge control valve cannot be controlled to a target flow rate, the air/fuel ratio is fluctuated, and further exhaust emissions are deteriorated. In particular, at the beginning of the operation restarted after keeping automobiles stopped for a long period of time, the amount of fuel adsorbed in the canister is large so that the amount of fuel vapor purged is large. Therefore, exhaust emissions is further deteriorated due to variations in characteristics of the purge control valve.